Voltage Regulator Testing (off car) - PeachParts Mercedes-Benz Forum

JamesDean · #1 09-03-2010, 02:12 PM

Hello everyone,

I'm having some trouble with my 300SD's charging system and I've been running some tests.

Alternator, new as of 2006.
voltage reg, new as of 01/2010

No load, system output decent 13+ volts.

Load (headlamps+ac+blower on high), system drops to 12.5v or lower.

I dont think the alternator itself is bad. I put a DMM on the battery in AC mode and only got .03-.05 volts AC. Meaning my diodes are functioning properly.

I pulled the voltage regulator, and was testing the transistor on it.

You can test a transistor like a diode. You should get a reading one way, but if you flip the probes you should get nothing the other.

Theres three pins on the transistor: collector, base, emitter. You are after the B-C, B-E connections for testing.

I believe...

The collector is connected to the casing of the transistor, and goes into a brush.
The base is the one that runs to one of the mounting screws for ground.
The emitter is the remaining one that goes into a brush.

When I tested my B-C, B-E resistances I got readings both ways. This might be because there might be a resistor/capacitor/diode in the circuit path that I can't see. But, I think the VR is just bad.

I was hoping someone could corroborate readings with me.

I dont have a new-erish vr to test.

TX76513 · #2 09-03-2010, 02:17 PM

James does this help? Place one of the ohm meter leads on the single connector on the Trio (Touch the other lead to each of the other three connectors, one at a time. The meter needle should either show continuity or no continuity, but should be the same for each of the three connectors. Now reverse the ohm meter leads and do the test again. The meter reading should now be reverse of the reading achieved on the first test (if the needle “pegged” on the first test, it should do nothing with the leads reversed)

JamesDean · #3 09-03-2010, 02:20 PM

When I tested I got continuity from b-c, c-b. b-e, e-b. I got continuity in both directions. So the meter reading did not show the reverse.

I'm probably going to order a new one regardless as its only $25

mak · #4 09-04-2010, 02:28 AM

it would be nice to have a tester to test the voltage regulator on the bench.wonder if someone has got one rigged up
mk

mpolli · #5 09-04-2010, 01:52 PM

I tested one on a bench one time. It was a Bosch type with the brushes built in. It was pretty easy although I needed a variable power supply. I had identified the +, -, and control pins (the control pin being the collector of the transistor. The idea is the transistor turns on when the voltage is below 14.4 (or whatever the regulator is set at) and turns off above. So I hooked a test light from + to control. When I changed the voltage, the light would turn on or off. My regulator was fine, but the brushes were worn down.

ps2cho · #6 09-04-2010, 02:00 PM

Just wanted to throw this out there....

Was running a similar issue last month. Battery held 12.6v overnight, but went flat during drive after about 20minutes. I checked voltages...
no load: 13.8v
load (lights + radio): 12.8v

I load tested battery, checked out...
Replaced VR....no change.

I ended up replacing the battery and the problem was fixed...hasn't returned since I guess the battery was functioning and holding a charge fine, but as soon as it had load on it, it couldn't handle it.

Since you have already replaced the alternator recently, I would say look at the battery. How old is it? Mine was 3 years old. Maybe buy a new one and if doesn't fix the issue return it.

mak · #7 09-05-2010, 09:27 AM

Now that is worthwhile info to check for a partially failing battery or substitute it for diagnosis purposes .
mak

Mike D · #8 09-05-2010, 10:03 AM

Don't forget. You can't have amps AND voltage. With a load applied the voltage should and will drop. The load is calling for more amps and the voltage will drop accordingly.

It is rare for the Bosch alternators to fail. It is not rare for a weak battery and or faulty earth connections to simulate an alternator problem. Many times I have seen someone change an alternator and claim that "fixed" the problem. In reality, the process of replacing the negative terminal on the battery (which they removed to isolate the battery) actually was the solution.

99.99% of all Bosch alternators merely require a quick swipe of the commuter with some fine emery cloth and replacement of the brush pack/ regulator to restore them to new condition. Bosch makes an adjustable regulator which is an exact fit. Tweak the little screw on the back to the wanted voltage. About $20 at any re-builders.

JamesDean · #9 09-05-2010, 12:33 PM

I'll double check the ground connections. The battery is good, it holds charge and was tested fine by the parts store here.